Heparin-induced thrombocytopenia (HIT) is a potentially catastrophic, antibody-mediated complication of heparin therapy caused by immunization against platelet factor 4 (PF4) complexed with heparin or other polyanions. HIT antibodies bind to PF4/heparin complexes on the platelet surface, resulting in platelet activation that leads to a platelet count decrease that can be accompanied by life-threatening thrombosis. This prothrombotic disorder can produce devastating thromboembolic complications, including ischemic limb necrosis, pulmonary embolism, myocardial infarction, and stroke.
Moderate thrombocytopenia is common in the clinical settings where heparin is administered and most cases are not caused by HIT. Differentiation of HIT from other potential causes of thrombocytopenia is a difficult diagnostic component in the evaluation of heparinized, thrombocytopenic patients and relies on a combination of a clinical assessment and laboratory investigation. Prompt diagnosis and management is critical to minimizing the risk of life-threatening thrombosis. Patients diagnosed with or suspected of suffering HIT must be taken off heparin and transitioned to an alternative non-heparin anticoagulant as quickly as possible.
The laboratory investigation of HIT is challenging and requires correlation between clinical symptoms and laboratory assays. The most common assay performed is a serologic assay that detects the presence of HIT antibodies without regard for their functional ability. Several serologic assays which are relatively easy to perform are available commercially and these assays are highly sensitive. The results of these assays have excellent negative predictive values and a negative result can be used to exclude HIT in all but the most compelling clinical circumstances. However, these assays suffer from low specificity and frequently give positive results in the absence of clinical HIT. A positive result, especially of low titer, does not differentiate between pathogenic antibodies and clinically irrelevant antibodies.
Another approach to measure HIT is to use an assay that measures platelet function, i.e., a functional assay. Functional assays that measure platelet activation by HIT antibodies in the presence of heparin are considered gold standard diagnostic laboratory tests due to their ability to detect the patient's underlying procoagulable state in those with true HIT. One functional assay is the measurement of serotonin released by platelets, i.e., the serotonin release assay (SRA). In this assay, sera from patients with heparin-induced thrombocytopenia (HIT) initiate platelet aggregation and secretion at therapeutic concentrations of heparin, but not at high concentrations of heparin. Generally, such assays measure radiolabeled serotonin released from platelets. However, due to complexity of performance, functional assays that use washed platelets are not widely available. Furthermore, serotonin release assays using washed platelets are typically performed using platelets that have been incubated with radiolabeled serotonin, and thus are accompanied by the drawbacks associated with using radioactive material.